1. Field of the Invention
The present invention relates to a method for satellite communications and, more particularly, to a satellite bus, its corresponding satellite subsystems, and a T&C that communicate by IP protocol.
2. Description of the Background
Satellites are proliferating for communication, surveillance, meteorology, navigation, etc. In each case a ground control system is needed for monitoring and controlling the satellite and its various payloads. The payloads are connected to a satellite bus, and commands are sent to the payloads via the satellite bus. For more information on the basics of satellite subsystems refer to Sellers et al., “Understanding Space: An Introduction to Astronautics”, McGraw-Hill Create (2005) ISBN 10: 0073407755).
Telemetry and science data downlinked from a satellite is typically transmitted to the mission operations center (MOC) and to its users and operators via ground stations and/or relay satellites. A telemetry and commanding system (T&C) is used at the MOC so that the satellite operator can send commands to the satellite, and view telemetry sent by the satellite. Commands sent by the satellite operator travel from the MOC to the ground station, and from the ground station to the relay satellite, and from the relay satellite to the actual satellite being commanded. Telemetry follows the reverse order, where the actual satellite will send telemetry and that telemetry is received by the ground station or the relay satellite and its ground station before reaching the MOC and the T&C. The operator would use the T&C system to monitor the state of health and operating limits of the various satellite subsystems and payloads. Aboard the satellite the following are the typical systems that are connected to and communicate over the satellite bus:                Electric power/distribution subsystem (EPS or EPDS): the hard- and software used to generate and distribute electrical power to the spacecraft, including solar arrays, batteries, solar-array controllers, power converters, electrical harnesses, battery-charge-control electronics, and other components;        Communication and Data Handing (C&DH): The electronics onboard a satellite that allow the satellite to receive commands from the user/operator and to send telemetry to the user/operator. The communication subsystem would consist of equipment such as: transmitters and receivers, transceivers, and antennas. The data handling electronics distribute and or store the necessary data.        Attitude/orbit control subsystem (AOCS): The devices used to sense and control the vehicle attitude and orbit. Typical components of the AOCS system include sun and Earth sensors, star sensors (if high-precision pointing is required), reaction or momentum wheels, Inertial Measurement Units (IMUs), Inertial Reference Units (IRUs), and the electronics required to process signals from the above devices and control satellite attitude;        Propulsion subsystem: Liquid and solid rockets or compressed-gas jets and associated hardware used for changing satellite attitude, velocity, orbit, or spin rate. Solid rockets are usually used for placing a satellite in its final orbit after separation from the launch vehicle. The liquid engines (along with associated plumbing lines, valves, and tanks) may be used for attitude control and orbit adjustments as well as final orbit insertion after launch;        Thermal-control subsystem (TCS): The hardware used to control temperatures of all vehicle components. Typical TCS elements include surface finishes, insulation blankets, heaters, and cryogenic coolers.        
Each satellite is designed around its payload, and each of the subsystems mentioned above help the payload complete its mission. The satellite bus or spacecraft bus is the general model on which multiple-production satellite spacecraft are built, and each satellite manufacturer typically provides a proprietary bus architecture.
Each of the subsystems mentioned above communicate over the satellite bus using prescribed data communications protocols. Traditionally proprietary protocols were used or variations on Asynchronous Transfer Mode (ATM). However, in the past decade satellite manufacturers have begun to embrace standardized protocols, allowing users to interact with standard computer PCs.
TCP/IP based network protocols are widely used today for various applications, such as turning a car on from your mobile device, commanding home security systems from your home device, and many more. TCP/IP is a set of protocols that allows anyone with a computer, modem, and an Internet service provider to access and share information over the Internet. TCP/IP enables cross-platform, or heterogeneous, networking. For example, a Windows NT/2000 network could contain Unix and Macintosh workstations or even networks mixed in it. TCP/IP also has the following advantages: 1) good failure recovery; 2) the ability to add networks without interrupting existing services; 3) high error-rate handling; 4) platform independence; and 5) low data overhead.
TCP/IP-based networking protocols can conceivably be used to command and control satellites and to allow for data transfer to and from satellites. However, TCP/IP does not perform well in networks having a large bandwidth-delay such as satellite links, and so it has not been very widely used by most of today satellite manufacturers. Another problem with TCP/IP is its weakness to recover from frequent losses in a wireless environment. In an IP network the sender sets a certain window for transmission of packets, and will cut back its window size if it encounters congestion. Any packet loss is a congestion indication and consequently it cuts back the window. Due to the high bit error rate in satellite links, such behavior is seen as congestion, which leads to a significant deterioration in TCP/IP throughput.
What is needed, is an IP-based satellite bus and method for satellite control in space. This would permit operations control on-orbit, in near real time within a secure system environment, with a dramatic increase in mission efficiency, an expansion of how much and what can be done on-orbit, and cost savings on future missions using TCP/IP-compliant spacecraft and payloads. The transport layer is part of the layered architecture of protocols in the network stack in the Internet Protocol Suite, and these protocols of the transport layer provide host-to-host communication services for applications. See, Maharaja, Rishabh, “Satellite Commanding, Controlling, and Data Transfer Concept using TCP/IP—From Classroom to Application” (2015)